Process for making neutral trisubstituted phosphates



Patented Sept. 2, 1947 PROCESS FOR, MAKING NEUTRAL 'mr- SUBSTITUTEDPHOSPHATES Russell L. Jenkins, Anniston, Ala., assignor to MonsantoChemical Company, St. Louis, Mo., a.

corporation of Delaware No Drawing. Application October 30, 1944, SerialNo. 561,162

The present invention relates to simple and mixed neutral phosphoricacid esters and to a novel process for producing same.

One object of the invention is to provide an improved process wherebyphosphoric acid esters of hydroxy hydrocarbon compounds and derivativesthereof may be produced in a state of high purity and in an economicaland efficient manner.

Another object is to provide a process for making trimethyl phosphateand other neutral watersoluble phosphoric acid esters wherein each stepis carried out in the substantial absence of water, thereby avoiding theproduct recovery problems inherent in the conventional phosphorusoxychloride process and thus making possible the production and recoveryof the ester products in substantially quantitative yields.

An additional object is to provide a process for making mixed neutralesters of phosphoric acid in a substantially pure form and in highyields, containing a minimum of undesirable by-products such as mixturesof simple and/or mixed neutral esters.

Other objects and advantages will be apparent to those skilled in theart as the description of the invention proceeds.

The process generally employed in the production of simple and mixedneutral esters of phosphoric acid consists essentially in reactingphosphorus oxychloride with hydroxy hydrocarbon compounds or mixturesthereof, quenching, and

'neutralizing the reaction mixture with sodium carbonate, and thenrecovering the esters from the neutralized product. It will be evident,however, from the considerations which immediately follow that thisprocess is not suitable for making soluble esters of phosphoric acidwhich hydrolyze readly in the presence of water.

For example, according to conversion calculations a high yield oftrimethyl phosphate is obtainable by the above procedure, but theseparation of this compound from the reaction mixture presents a seriousproblem since it is so soluble in water. Attempts have been made torecover this compound from the reaction mixture by extraction withvarious solvents, but, so far as applicant is aware, they have beenunsuccessful. Steam distillation has also been resorted to, but thisresults in excessive hydrolysis of the ester and requires the handlingof such large volumes of liquid that this method of operation isimpractical. Etl'orts have also been made to solve this problem byneutralizing the reaction mixture in 13 Claims. (Cl. 260-461) 2 theabsence of water, but it was found that this reaction is so slow as tobe of no practical value.

As applied to the production of mixed neutral esters, the above methodis subject to the disadvantage that some hydroxy hydrocarbon compoundsare more reactive than others and consequently the reaction productcontains a mixture of simple esters and/or neutral mixed esters. Thismakes it necessary to separate the esters not only from the reactionmixture but also from each other with the result that the cost ofoperating the process is materially increased and the yield of thedesired ester is substantially decreased.

As a modification of the foregoing process it has also been proposed tomanufacture phosphoric acid esters by reacting phosphorus oxychloridewith metal derivatives of hydroxy hydrocarbon compounds such as sodiummethylate, sodium ethylate, etc., but this procedure'has the addeddisadvantage of producing three moles of a metallic chloride for eachmole of phosphorus oxychloride, thus requiring the separation ofexcessive amounts of this impurity from the product.

Now I have developed an economical and commercially feasible process forproducing phos phoric acid esters of the above type which has none ofthe objectionable features enumerated above. This process involvesreacting phosphorus trichloride with a hydroxy hydrocarbon compound toform a disubstituted hydrogen phosphite, chlorinating the crude reactionmixture thus obtained to produce the corresponding disubstitutedchlorophosphate and then reacting the latter with an esterifyingcompound to form a simple or mixed neutral ester depending upon thehydrocarbon compound and the esterifying compound employed. Thereactions which take place in this process are represented by thefollowing equations:

(l) ROH PClz (RO)2POH RC1 21101- (2) (ROhPOH Ch (RO)zPOCl HCl v RQ (3)(ROMPOCI (RO)yX -v RO 7P=O XCI where R is an aliphatic or an alicyclicradical, R is a hydrocarbon radical, X is a hydrogen atom or a metalselected from the group consisting of alkali metals, calcium, barium,strontium, magnesium and aluminum, and Y is a positive number having avalue corresponding to the valence of X.

For a more detaileddescription of the above process for producing simpleand mixed neutral,

esters of phosphoric acid, reference is made to the following exampleswhich are illustrative of the preferred embodiments of the presentinvention.

Exsmrtr: I

500 c. c. of anhydrous benzene and 372 c. c. (9.2 moles) of anhydrousmethyl alcohol were introduced into a three neck flask which wasequipped with a stirrer, a fritted glass gas disperser, a droppingfunnel and a downward condenser. The solution thus obtained was cooledin an ice-salt bath to C. and then. the system was subjected to apressure of about 700 millimeters of mercury. While the system was underthis pressure, 262 c. c. (3 moles) of phosphorus trichloride was addedslowly with vigorous stirring in order to maintain the temperature ofthe reaction below 5 C., the time of addition being approximately 1hour.

As soon as the reaction had been completed, the system was subjected toa pressure of from 50 to 200 millimeters of mercury, whereupon anhydrouschlorine was gradually bubbled into the mixture for one hour and aquarter at such a rate that the reaction temperature was maintainedbelow 5 C. The end point of the chlorination was indicated by a changein the color of the solution to a permanent green and also by a rapiddrop in temperature. At this point the chlorination was stopped, 200 c.c. of benzene was added to the mixture and, while the system wassubjected to a pressure of from 50 to 200 millimeters of mercury toremove the residual hydrogen chloride, the temperature was allowed torise slowly over a period of two hours. At the end of the first hourheat was applied to distill the benzene, but the temperature was notpermitted to exceed C. At the end of this period most of the benzenehadbeen distilled and the byproduct HCl removed, leaving crude dimethylchlorophosphate as a residue.

The crude chlorophosphate product thereby obtained was added withstirring to a sodium methylate solution containing phenolphthalein,which had been previously cooled on an ice bath. During this operationthe reaction temperature was maintained at from 10 to 15 C. The additionof chlorophosphate was continued until the indicator changed colorshowing that the mixture was on the acid side. The sodium chloride whichprecipitated as a result of this reaction was filtered off, washed withdry benzene and the filtrate and washings distilled at a temperature of55 to 60 C. and a pressure of 3 to 5 millimeters of mercury. Thedistillate obtained was refractionated to purify the product at atemperature of 67-68 C. and under a pressure or 6 to 7 mm. of mercury.388 grams of trimethyl phosphate was obtained, representing a yield of80% of theory, basis PCla.

EXAMPLE II In a three neck flask of suitable size fitted with stirrer,fritted glass gas bubbler, dropping funnel and downward condenser, 1818c. c. of anhydrous benzene and 1272 c. c. (31.4 moles) of methyl alcoholwere mixed and cooled in an ice-salt bath to 0 C. The system wasthereupon subjected to a pressure of about 700 millimeters of mercuryand 909 c. c. (10.7 moles) of phosphorus trichloride was added for aperiod of three hours, with vigorous stirring. through the droppingfunnel at the fastest rate that would allow the temperature to bemaintained below 5 C. At the end of the reaction, a vacuum correspondingto an absolute pressure of 50 to 200 millimeters of mercury was appliedand then chlorine, which had been dried by means of sulfuric acid, wasbubbled into the mixture for a period of four and one quarter hours, therate of the chlorine addition being so controlled as to maintain thereaction temperature below 5 C. The end point of the chlorination wasindicated by the solution tuming a permanent greenish color togetherwith a rapid drop in temperature as the reaction ceased. At this pointthe chlorination was discontinued.

The temperature of the chlorinated product was then maintained between 0and 5 C. for about 3 hours under an absolute pressure of 50 to 200millimeters of mercury. This was followed by. a 3 hour evacuation at astill lower pressure by means of a Kinney large capacity pump, duringwhich period the temperature of the reaction was slowly raised to amaximum of 30 C. This latter operation effected a distillation of mostof the benzene which carried with it all or substantially all of theresidual hydrogen chloride and left behind a residue of crude dimethylchloro phosphate.

This crude product was added with vigorous stirring to a previouslyprepared cold sodium methylate solution containing a small quantity ofphenolphthalein indicator. The temperature of the reaction wasmaintained at 15 to 20 C. and the addition of the crude chloroph'osphatewas continued until the indicator changed color showing that the mixturewas on the acid side. The precipitated sodium chloride was filtered off,washed with dry benzene and the filtrate and washings distilled andrefractionated as in Example I. A distillate was collected consisting of2.75 lbs. of trimethyl phosphate, a y eld of 83% of theory, basis PO13.

The trimethyl phosphate obtained in Examples I and IIpossessed thefollowing physical propert es:

Specific gravity 1213-1214 at 25/15.5 C. Boiling point 189 C. at 760 mm.Vapor pressure 0.52 mm. at 22 C.

- Pour points Liquid at -70 C.

Melting point Liquid at 70 C. Solubility in water Infinitely In view ofthe fact that trimethyl phosphate i extremely soluble in water and isalso readily hydrolyzed thereby, the reaction is preferably carried outin the substantial absence of water, otherwise the product yield will bematerially reduced. This precaution is also important in the productionof other water-soluble phosphoric acid esters.

The various steps of the present process will now beconsidered indetail.

Step I In the first step or my process phosphorus trichloride is reactedwith an aliphatic alcohol in the presence of benzene, carbontetrachloride or other suitable inert solvent to form the correspondingdialkyl hydrogen phosphite. The optimum temperature for this reactionvaries with the number of carbon atoms contained in the alkyl chain ofthe alcohol. For the dimethyl compound the reaction temperature shouldfall substantially in the range of from -15 C. to 5 C. but higher orlower temperatures may also be employed if desired and when the reactionis executed at a temperature below 0 C., the addition of an inertorganic solvent may be omitted.

With regard to the quantities of reagents employed in the initial stepof my process, an excess of the theoretical amount of alcohol requiredto form the desired phosphate should be used. Generally from 1 to 10%excess is satisfactory, but more or less alcohol may be employed so longas the theoretical requirements are met.

Step II In the production Of dialkyl chlorophosphates in accordance withthe second step of my process, the dialkyl hydrogen phosphite ischlorinated in the presence of an inert organic solvent of the abovetype to produce a crude mixture containing the corresponding dialkylchlorophosphate. In this reaction the chlorination temperature varieswith the dialkyl hydrogen phosphite being treated and also with theamount and type of solvent used. In chlorinating dimethyl hydrogenphosphite the reaction should take place at a temperature substantiallywithin the range of -5 to 8 C. and preferably at a. temperature below 5C.

The use of reduced pressure in the chlorination step is desirable as itaids in controlling the temperature of the reaction and at the same timefacilitates the removal of undesirable vapors, but

this method of operation is not essential as very high yields of dialkylchlorphosphates are obtainable at atmospheric pressure if efliicentcooling means are provided.

Step III The removal of hydrogen chloride in the third step of myprocess is desirably carried out by adding benzene, carbon tetrachlorideor other suitable inert solvent which decreases the solubility of theHCl in the mixture, and then sweeping out the acid by solvent vaporsproduced by distillation, preferably by vacuum distillation. Thesweeping operation is initiated at a relatively low temperature which igradually increased until all or g has been removed, care being taken toavoid raising the temperature to a point where substantial decompositionof the chlorophosphate takes place.

For example, when removing hydrogen chloride from mixtures containingdimethyl chlorophosphate, the sweeping operation preferably begins at atemperature of about 0 C. and ends at a temperature of 30 C. However,satisfactory results have been obtained at temperatures within the rangeof --15 C. and 50 C.

It should be understood that the starting temperature of the sweepingoperation will vary with the amount of hydrogen chloride present in thecrude chlorophosphate mixture and the chlorophosphate mixture beingsweetened. If substantial amounts of hydrogen chloride are present, thenastarting temperature of -15 C. or a lower temperature should beemployed, but if only relatively small amounts of hydrogen chloride arepresent, then the sweetening step may be initiated at somewhat hightemperatures.

Step IV In preparing trimethyl phosphate from dimethyl chlorophosphatein accordance with the fourth step of my process, the reaction iscarried out at a temperature substantially in the range of from 10 to 20C. and preferably in the rangeof flom to C. since in the latter rangethe sodiumv chloride precipitates in a form in which it may be easilyfiltered. It is also desirable, although not absolutely essential, toadd the dimethyl chlorophosphate to the soduim methylate rather than theconverse of this operation as I thereby obtain a material increase inthe yield of the product.

Instead of employing sodium methylate, the

substantially all of the acid I reaction may be carried out using methylalcohol, alone or in combination with ammonia or a nitrogenous base.When this is done, the reaction should likewise be eflected at atempera-'- ture within the above range, but satisfactory results arealso obtainable at somewhat higher or lower temperatures.

As an alternative to the foregoing procedures, the dimethylchlorophosphate may be reacted with substantially one half of a mole ofcalcium barium, strontium or magnesium methylate or about one third of amole of aluminum methylate.

When producing phosphoric acid esters from dialkyl or di-alicyclicchlorophosphates and other hydroxy hydrocarbon compounds or the metalderivatives thereof, the reaction conditions will vary somewhat with thecompounds employed and in view of the very large number of estersobtainable by the present process, no attempt has been made to set forththe conditions required for the production of each ester.

Step V In the fifth and final step of my process for producing trimethylphosphate, the sodium chloride is filtered oil and the filter cake iswashed with methanol or benzene. Th'e filtrate and washings are combinedand vacuum distilled to remove the solvents and the residue is distilledat to C. under a vacuum corresponding to 3 to 5 millimeters of mercury.The distillate thus obtained is refractionated at a pressure of 6 to '7millimeters of mercury and at a temperature of from 67' to 68 C. toyield asubstantiaily pure product.

If methyl alcohol is employed in the fourth step of my process, then thereaction product is vacuum distilled in the presence or absence ofbenzene or other high boiling solvent to remove the byproducts, meth'ylchloride and hydrogen chloride, after which the residue is distilled andrefractionated to recover trimethyl phosphate in the manner described inExamples I, and II.

However, when dialkyl or di-alicyclic chlorophosphates are reacted withhydroxy hydrocarbon compounds to form water insoluble esters, thehydrogen chloride contained in the neutral ester mixture is preferablynot removed by distillation; it is extracted by means of a solution ofsodium hydroxide or sodium carbonate and after separating the aqueouslayer, the esters are recovered from the non-aqueous layer-bydistillation.

If methyl alcohol and ammonia or a nitrogenous base of the typeindicated above are employed, then in this case it is also unnecessaryto remove the hydrogen chloride from the neutral ester mixture bydistillation as it is absorbed by these nitrogen compounds to producethe corresponding salts which are then separated from the 'product byfiltration. The filtrate thus obtained is distilled and retractionatedto recover trimethyl phosphate in a substantially pure form. The sameprocedure is employed when other hydroxy hydrocarbon compounds are usedin combination with ammonia or nitrogenous bases in the production ofother phosphoric acid esters.

When calcium methylate, magnesium methylate, barium methylate, strontiummethylate and aluminum methylate are reacted with dimethylchlorophosphates or other dialkyl (or di-alicyclic) chlorophosphates,the procedure for recovering the esters is identical with the sodiummethylate process except that instead of sodium chloride, calcuim,barium, strontium, mag- 1. Simple and mixed neutral phosphoric acidesters of aliphatic alcohols containing up to 20 or more carbon atoms.

2. Simple and mixed neutral phosphoric acid esters of cycloaliphaticalcohols.

3. Mixed dialkyl-alicyclic esters of phosphoric acid. a

4. Mixed di-alicyclic-alkyl esters' of phosphoric acid.

5. Mixed dialkyl-aryl esters of phosphoric acid.

6. Mixed di-alicyclic-aryl esters of phosphoric acid.

The compounds produced in accordance with this invention have thefollowing general formula:

wherein R is an aJwl radical containing up to 20 or more carbon atoms oran alicyclic radical 2. The process defined in claim 1 wherein themethylate employed is sodium methylate.

3. The process defined in claim 1 wherein the methylate employed is analkaline earth methylate.

4. The process defined in claim 1 wherein the methylate employed isaluminum methylate.

5. The process for manufacturing trimethyl phosphate, which comprisesreacting about 3 molecular proportions of methyl alcohol with about 1molecular proportion of phosphorus trichloride to form a crude productprincipally containing dimethyl hydrogen phosphite, chlorinating saidproduct at a temperature of from about -5 C. to about 8 C. to producedimethyl chlorophosphate and thereupon reacting said chlorophosphatewith sodium methylate in the substantial absence of water to formtrimethyl phosphate, said methyl alcohol and said phosphorus trichloridebeing the sole reactants in said initial step of the process.

6. The process defined in claim 5 wherein the reaction betweenmethylalcohol and phosphorus trichloride is carried out at a temperature offrom 15 C. to 5' C.

7. The process defined in claim 5 wherein the reaction between methylalcohol and phosphorus trichloride is carried out at a temperature below0 C.

8. The process defined in claim 5 wherein the dimethyl hydrogenphosphite is chlorinated at a and R is an aryl radical. a cycloaliphaticradi-.

cal or an alkyl radical containing up to 20 or more carbon atoms.

These compounds are useful as addition agents for lubricating oils,lacquers, varnishes, etc., and also as plasticizers for celluloseesters, cellulose ethers and synthetic resins.

Reference is hereby made to application, S. N. 520,100, filedin the nameof Edgar E. Hardy and Gennady M. Kosolapofl, for a more detaileddescription of the method for producing the dialkyl cholorophosphatesused in the production of neutral phosphoric acid esters in accordancewith this invention.

Where the expression hydroxy hydrocarbon compounds is used in thespecification and claims,-it is to be understood that both thesubstituted and unsubstituted hydroxy hydrocarbon compounds and themetal derivatives thereof are contemplated.

As many widely different embodiments of this invention-may be madewithout departing from the spirit and scope thereof, it is to beunderstood that I do not limit myself to the specific embodimentsthereof except as defined in the following claims.

What I claim is:

1. The process for manufacturing trimethyl phosphate, which comprisesreacting at a temperature of about 15 C. to about 5 C. substantially 3molecular proportions of methyl alcohol with about 1 molecularproportion of phosphorus trichloride to form a crude product principallycontaining dimethyl hydrogen phosphite, chlorinating said product toproduce dimethyl chlorophosphate and thereupon reacting saidchlorophosphate with a compound selected from the group consisting ofmethyl alcohol and methylates in the substantial absence of Water toform trimethyl phosphate, said methyl alcohol and said phosphorustrichloride being the sole reactants in said initial step of theprocess. 1

temperature below 5 C.

9. The process defined in claim 5 wherein the dimethyl hydrogenphosphite is chlorinated under reduced pressure and at 'a temperaturebelow 10. The process defined in claim 5 wherein the reaction betweendimethyl chlorophosphate and sodium methylate is carried out at atemperature of from 10 C. to 20 C.

11. The process for manufacturing trimethyl phosphate, which comprisesreacting about 3 molecular proportions of methyl alcohol with about 1molecular proportion of phosphorus trichloride in the presence of aninert organic solvent and at a temperature of from 15 C. to 5 C. to forma crude mixture principally containing dimethyl hydrogen phosphite,chlorinating said crude mixture at a temperature of from -5 C. to 8 C.while under reduced pressure to produce a crude chlorinated productprincipally containing the corresponding dimethyl chlorophosphate andhydrogen chloride, removing said hydrogen chloride by distilling saidcrude prod uct under reduced pressure and at a temperature of from -15C. to 50 C. introducing the resulting crude dimethyl chlorophosphateresidue into a sodium methylate solution and thereby causing thesecompounds to react together in the substantial absence of water toproduce a reaction product containing trimethyl phosphate and sodiumchloride and then recovering said trimethyl phosphate from said reactionproduct by distillation, said methyl alcohol and said phosphorustrichloride being the sole reactants in said initial step of theprocess.

12. The process for manufacturing trimethyl phosphate, which comprisesproducing acrude mixture principally containing dimethyl hydrogenphosphite by reacting substantially 3 molecular proportions of methylalcohol with substan- 9 lute pressure on said system to a value of from50 to 200 millimeters of mercury, then chlorinating said crude mixtureat a temperature below 5 C. to form a crude product principallycontaining dimethyl chlorophosphate and hydrogen chloride, removing saidhydrogen chloride by ini- I tially distilling said crude product at atemperature of from C. to C. for about 3 hours while under an absolutepressure of from 50 to 200 millimeters of mercury and then continuingsaid distillation for an additional 3 hours at a lower absolute pressurewhile allowing the temperature of the crude product to gradually rise toa maximum of 30 C. introducing the resulting crude dimethylchlorophosphate residue into a sodium methylate solution and therebycausing these compounds to react together in the substantial absence ofwater to produce a crude mixture containing trimethyl phosphate and thenseparating said trimethyl phosphate from said crude mixture bydistillation, said methyl alcohol and said phosphorus trichloride beingthe sole reactants in said initial step of the process.

13. The process for manufacturing trimethyl phosphate, which comprisesproducing a crude mixture principally containing dimethyl hydrogenphosphite by reacting in the presence of benzene about 3 molecularproportions of methyl a1- cohol with about 1 molecular proportion ofphosphorus trichloride at a temperature below 5 C. and while the systemis under an absolute pressure 01' about 700 millimeters of mercury,reducing the absolute pressure on said system to a value within therange of from 50 to 200 millimeters of mercury, then chlorinating saidcrude mixture at a temperature below 5 C. to form a product principallycontaining dimethyl chlorophosphate and hydrogen chloride, distillingsaid product at an absolute pressure of from 50 to 200 millimeters ofmercury to remove said hydro- ,0

gen chloride while permitting the temperature of said product to risegradually to a point not exceeding 30 C., introducing the resultingcrude dimethyl chlorophosphate residue into a sodium methylate solutionand thereby causing these compounds to react together in the substantialabsence of water to produce a crude product containing trimethylphosphate and then recovering said trimethyl phosphate by distillingsaid crude' product at a temperature of from about C. to C. while underan absolute pressure of from about 3 to 5 milimeters of mercury, saidmethyl alcohol and said phosphorus trichloride being the sole reactantsin said initial step of the process.

RUSSELL L. JENKINS.

' REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,023,758 Raschig Apr. 16, 19121,844,408 Nicolai Feb. 9, 1932 1,869,768 Nicolai Aug. 2, 1932 2,005,619Graves June 18, 1935 2,176,416 Britton Oct. 17, 1939 2,409,039 Hardy etal. Oct, 8, 1946 FOREIGN PATENTS Number Country Date 566,514 GermanyDec. 17, 1932 OTHER REFERENCES Gerrard, Jour. Chem. Soc. (London), 1940,pp. 1464-1469.

Beilstein, "Handbuch der Org. Chem.," vol. I, 4th ed., p. 332.

